In recent years, flat panel displays have become more and more integrated into daily life. Displays can be distinguished into two main types: cathode-ray tube displays and liquid crystal displays. Among them, the liquid crystal display (LCD) has advantages of light weight, small size, full-color display, less radiation, digitization, high definition, energy-saving and the like compared with the traditional cathode-ray tube displays. As a result, the liquid crystal display has definitely become the main trend in display technology.
The composition and the luminous principle of a liquid crystal display are very different from a cathode-ray tube. Briefly, a liquid crystal display controls the geometry of liquid crystals by using an electric field to alter the transport pathway and the phase of light, and cooperates with a polarizer to attain the effect of light-and-shade. By further cooperating with a drive circuit and color filters, the liquid crystal panel can display both gray scale and color images. The process for fabricating a liquid crystal display panel can be basically divided into an array and a color filter process, a liquid crystal cell process, and a module process.
For example, the method for fabricating a thin film transistor-liquid crystal display (TFT-LCD) commonly seen can be divided into three main processes: the first is a process for creating an array and a color filter for driving and creating signals; the second is a liquid crystal cell process for controlling, filling and sealing a liquid crystal; and the third is a module process for fabricating a polarizer, a back light module and the liquid crystal cell. Among them, a liquid crystal alignment technique in the liquid crystal cell process particularly plays a key role. In addition to playing a major role in controlling the arrangement and the orientation of liquid crystals, the liquid crystal alignment process further relates to high-quality display properties such as visual angle, response speed, contrast ratio, color expression, and so on.
The term “liquid crystal alignment” means to give liquid crystal molecules a direction, and the purpose is to make all or part of the liquid crystal molecules have a unified and uniform orientation or a fixed directional arrangement. The object of applying liquid crystal alignment in a liquid crystal display is that all or part of the liquid crystal molecules must have a synchronized and consistent motion when an electric field acts and drives the motion of the liquid crystal molecules, so that the action of displaying can be quick and uniform. Accordingly, there is a need of an alignment technique to attain the objective above. The alignment techniques commonly seen today are divided into a rubbing alignment technique and a non-rubbing alignment technique.
Rubbing alignment technique is a popular alignment technique used in the industry now. As shown in FIG. 1, basically, a substrate 1 such as an ITO glass substrate is set on a platform and moved unidirectionally to fix a Rayon cloth 15 having short and meticulous filaments onto an outside surface of a roller 13. The roller 13 is rotated with a speed of several hundreds revolutions per minute (rpm), so that the Rayon cloth 15 on the outside surface thereof is pressed into the material of an alignment layer 11 (for example, polyimide (PI)), which is preformed on the substrate. The surface of the alignment layer is rubbed with the short filaments at high speed to perform the rubbing process. After rubbing, the molecular arrangement on the surface of the material of the alignment layer 11 is under control and the molecules are arranged regularly along the direction of rubbing. Accordingly, the liquid crystals to be filled and sealed subsequently can also be arranged in alignment direction due to the force of interface.
The advantages of the rubbing alignment technique include a very short operation time for rubbing in a fixed direction, capability of being operated at normal temperature and excellent mass-production property. Thus, the rubbing alignment technique has continued to be used in the process for thin film transistor-liquid crystal displays in the fourth-generation LCD factories today. However, as progress is made towards the technical aims of high luminosity, large size, wide visual angle, and so on, the rubbing alignment technique has many problems that are difficult to solve. For example, the problems of dust pollution, residual static charge, rubbing defects and the like that are caused by rubbing a thin film in the rubbing process can easily result in reduced process yield and poor reliability. Therefore, under the demand for higher process yield for large-size liquid crystal panels in the fifth- and sixth-generation factories in the future, the utilization of a non-rubbing alignment technique is very desirable and, as such, an area of research and development. Presently, three types of non-rubbing techniques have been developed: a photo alignment technique, an ion beam alignment technique, and a plasma beam alignment technique.
The materials used in the photo alignment technique are mainly based on a high molecular thin film such as polyimide (PI). The high molecular thin film is irradiated by an ultraviolet light source having anisotropic energy, so that anisotropic photopolymerization, photoconvertion or photocracking occur in the high molecular structure on the surface of the thin film. As a result, an anisotropic van de Waal's force is generated on the surface of the thin film to induce the arrangement of the liquid crystal molecules. A linear polarized ultraviolet light source is mainly used in the ultraviolet photo alignment method. In this method, the ultraviolet light source is polarized by using a polarizer. Because the anisotropic energy of the ultraviolet light source is high and capable of inducing the anisotropic photoreaction to occur effectively, the ultraviolet light source is widely used in the study of photo alignment materials. The photo alignment technique is nearly ten years old and has the feature of good uniformity. However, there are technological bottlenecks such as anchoring energy, image sticking and the like that remain to be solved. Moreover, the problems of light bulb life and light flicker when using an exposure machine severely impact the stability.
The procedure for the ion beam alignment process is to bombard an inorganic or organic alignment thin film material with an ion beam at a determined angle. An anisotropic structure is created on the surface of the thin film material by selective bond cleavage, and there is an alignment effect to the liquid crystals. The design frameworks for ion beam alignment apparatuses are similar and generally include a vacuum chamber, an ion source, an electrical neutralizer for neutralizing ions and a movable and rotatable platform for setting a glass substrate. As disclosed in U.S. Pat. No. 6,665,033B2, IBM Corporation has provided an ion beam alignment apparatus that uses a Kaufman-type ion gun as an ion source. The method for creating the ions is that plasma is first generated within the ion gun, and then part of the cations in the plasma pass through a pinhole on a plate; the cations are attracted by the negative potential of an acceleration electrode and shoot from the ion gun with high speed, thus generating an ion beam to be used for the alignment process. To avoid excessive charge accumulating in the alignment film, the ion beam should be subject to charge neutralization with electrons excited by a hot filament, so that the alignment treating process can be carried out on only the alignment film.
Because there is a need for using high vacuum equipment and static charge elimination equipment in the process of ion beam alignment, the cost is very expensive. Furthermore, the creation of the vacuum needs a lot of time and is restricted by the size of the equipment. For larger-sized panels, it needs specially-made equipment that is extremely expensive. In addition, the problem of the relatively short life of the ion gun used in the process has not been overcome. Therefore, the ion beam alignment technique presently remains in the stage of laboratory development.
Plasma beam alignment is also called particle beam alignment. This plasma beam comprises ions, electrons, neutral gases, an ultraviolet ray and ultraviolet light. The most original concept of the plasma beam alignment comes from the short-range close drift technology in Soviet space research. Satellite technology was developed actively by the Soviet Union during the Cold War between the United States and the Soviet Union. Under the stringent demand of dynamic control of a satellite, the anode layer thruster (ALT) was developed, and the concept thereof has been extended to display technologies.
In the plasma beam alignment technique, a plasma source is generated by a direct current plasma system. Ion groups in the plasma are driven by a high electric field with positive bias created by a positive electrode, and an anode layer is generated to begin the alignment mechanism. Furthermore, the alignment film is subjected to surface modification treatment with the plasma. In addition to completely avoiding static charge generation and dust pollution, the plasma beam alignment technique has several other advantages: the alignment film treated by the plasma beam has the properties of photostability, alignment stability and the like; the plasma beam alignment technique can adjust the distribution range of the pre-tilt angle; and the expression of azimuthal anchoring energy can attain the same level as that of the photo alignment technique.
However, all of the plasma beam alignment techniques that have been disclosed in publications or patents use vacuum plasma equipment and thus the cost is very expensive. Furthermore, the creation of vacuum needs a lot of time and is restricted by the size of the equipment. For larger-sized panels, specially-made equipment is needed that is extremely expensive and occupies space for the equipment and the like. Thus, the plasma beam alignment technique also remains in the stage of laboratory development and hasn't attained industrial applicability.
Therefore, overcoming the existing problems disclosed in the prior arts is quite desirable.